This invention relates to a process for producing a polymer (in particular, a block copolymer) which is contaminated with little impurities and usable as a base polymer in the fields of films, adhesives, pressure-sensitive adhesives, molded articles, surface-coating agents, coatings and the like, additive components such as compatibilizing agents, softeners, dispersants and tackifier resins, mold release agents and release agents (or backside treatment agents).
It has been a practice to use various polymers, which are produced by radical polymerization, ionic polymerization and polycondensation reactions, for various purposes including a base polymer in the fields of films, adhesives, pressure-sensitive adhesives, molded articles, surface-coating agents, coatings and the like, additive components such as compatibilizing agents, softeners, dispersants and tackifier resins.
Among the polymerization methods as described above, the radical polymerization method has been widely utilized industrially. This is because this method is applicable to various types of monomers and less restricted in operation (i.e., the polymerization procedure should be performed in an inert gas such as nitrogen but it is unnecessary to control moisture, etc. in this method). Recently, a technique of living radical polymerization method, which falls within the category of the radical polymerization method, is developed. In this method wherein a monomer is polymerized by using a polymerization initiator in the presence of a transition metal and its ligand, the molecular weight and molecular weight distribution of the polymer can be more accurately controlled than in the usual radical polymerization method. In addition, a block copolymer can be synthesized thereby.
Accordingly, attempts have been made in various fields to control the block structure or functional groups of copolymers by the living radical polymerization method as described above so as to improve the properties thereof. In JP-A-11-236428, for example, it is discussed to improve the overall properties of pressure-sensitive adhesives by changing the compositions or molecular weights of copolymers and thus controlling the balance among adhesion, tackiness and holding power. On the other hand, JP-A-2000-72815 proposes materials which are modified by introducing acryloyl group into the terminal and thus can be cured by light irradiation or electron radiation. (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d.)
In the living radical polymerization method as described above, however, a transition metal and its ligand employed as a polymerization activator remain in the polymer. Even though the polymer has been homogenized by filtration or the like, the transition metal and its ligand dissolved in the polymer still remain therein in a large amount as impurities and thus cause troubles, for example, the coloration of molded articles such as films and the decrease in the mechanical properties of molded articles such as films. Moreover, there sometimes arise additional problems that, in case of crosslinking this polymer, the above-described impurities inhibit the crosslinking reaction and thus the crosslinkage cannot proceed well, or in case of using the polymer as a coat material such as an adhesive or a coating, these impurities migrate onto the surface of an adherend and thus cause the stain of the adherend.
JP-W-10-509475 discloses a process for producing a polymer comprising the step of performing atom- or group-transfer radical polymerization by using a transition metal and its ligand, and the step of isolating the thus formed polymer. In this process, the polymer is precipitated from an appropriate solvent, filtered and washed in the isolation step thereof. Therefore, there arise some problems, for example, need of a large amount of the solvent and complicated procedures and a decrease in the yield of the polymer. (The term xe2x80x9cJP-Wxe2x80x9d as used herein means an xe2x80x9cunexamined published International patent applicationxe2x80x9d.)
Under these circumstances, the invention aims at, in producing a polymer by the living radical polymerization method, reduce the amount of impurities consisting of a transition metal and its ligand employed as a polymerization activator to thereby give a polymer which is free from any problems such as the coloration or the decrease in the mechanical properties of molded articles such as films, crosslinkage failure or migration.
To achieve the object as described above, the inventors have conducted intensive studies and consequently found out that the content of a transition metal and its ligand, which are employed as a polymerization activator, in a polymer can be surely reduced and thus a polymer which is free from any problems such as the use of a large amount of the solvent, the remarkable decrease in the yield of the polymer, the coloration or the decrease in the mechanical properties of molded articles such as films, crosslinkage failure or migration can be easily produced by, after the completion of the formation of the polymer by the living radical polymerization method, eliminating the transition metal and its ligand which are employed as a polymerization activator, contained in the polymer while regulating the viscosity of the polymerization reaction mixture product to a definite level or below, and carrying out the elimination in two steps comprising the former step of eliminating the transition metal and its ligand insoluble in the polymer and the latter step of eliminating the transition metal and its ligand dissolved in the polymer (in particular, using a centrifuge and an ion exchange resin respectively in the former step and in the latter step), thereby completing the invention.
Accordingly, the invention relates to a process for producing a polymer comprising: a) the step of performing a polymerization reaction by using a monomer, a transition metal and its ligand and a polymerization initiator to form a polymer; and b) the step of eliminating the transition metal and its ligand contained in the polymer under the condition of a viscosity of 100 Paxc2x7s or below, characterized in that the step b comprises: b1) the former step of eliminating the transition metal and its ligand insoluble in the polymer; and b2) the latter step of eliminating the transition metal and its ligand dissolved in the polymer. In particular, the invention relates to a process which further comprises c) the step of eliminating the unreacted monomer after the step a, in addition to the step b, and a process wherein the combination of the transition metal and its ligand is a Cu+1-bipyridine complex.
The invention further relates to the process for producing a polymer as described above wherein the step b1 comprises the step of eliminating the transition metal and its ligand insoluble in the polymer by using a centrifuge; the process for producing a polymer as described above wherein the step b2 comprises the step of eliminating the transition metal and its ligand dissolved in the polymer with the use of an ion exchange resin; the process for producing a polymer as described above wherein no solvent is employed in any step; the process for producing a polymer as described above wherein the content of the transition metal and its ligand in the polymer is regulated to 30 ppm or below; the process for producing a polymer as described above wherein the number-average molecular weight of the polymer ranges from 3,000 to 300,000; and the process for producing a polymer as described above wherein the polymer is a block copolymer of the A-B type, the A-B-A type, the A-B-C type or the like.
The invention further relates to the process for producing a polymer as described above wherein the polymer is a block copolymer being tacky at room temperature which is composed of at least two blocks of a non-elastomeric polymer block A having a glass transition temperature of 20xc2x0 C. or above and an elastomeric polymer block B comprising a (meth)acrylate polymer bonded thereto. The invention furthermore relates to a pressure-sensitive adhesive composition characterized by comprising, as the main component, a block copolymer being tacky at room temperature, which is produced by the process as described above, of the A-B type, the A-B-A type or the B-A-B type.
Moreover, the invention relates to the process for producing a polymer as described above wherein the polymer is a block copolymer composed of at least two blocks of a polymer block A having a glass transition temperature of 0xc2x0 C. or above and a (meth)acrylate polymer block B having a glass transition temperature of lower than 0xc2x0 C. bonded thereto. The invention further relates to a film-forming material characterized by comprising, as the main component, a block copolymer, which is produced by the process as described above, of the A-B type, the A-B-A type or the B-A-B type; and a coating composition characterized by comprising the same as the main component.